Mass mortality of cultured Japanese whiting (Sillago japonica) associated with Miamiensis avidus: Temperature-dependent parasite proliferation and host susceptibility.

Mass mortality events occurred in cultured Japanese whiting (Sillago japonica) in Japan between 2018 and 2025. Dead fish exhibited severe skin erosion, ulceration, and hemorrhaging, with numerous ciliates observed in affected lesions. Histopathological examination revealed epidermal detachment, ulceration, and extensive parasite infiltration into connective and muscle tissues. The causative agent was identified as Miamiensis avidus through 18S rRNA gene analysis. PCR detected parasite DNA in multiple organs, confirming systemic infection. To clarify the role of temperature in disease occurrence, in vitro proliferation and in vivo experimental infection trials were conducted at different temperatures. In vitro, M. avidus exhibited optimal growth at 20-25 °C, while growth was markedly reduced at ≤15 °C and ≥ 30 °C. Experimental infection trials at 12, 18, 24, and 28 °C showed that high mortality occurred at 12 °C and 28 °C in both control and infected groups, suggesting that these temperatures impose significant thermal stress. At 18 °C, mortality was observed only in infected fish, indicating that Japanese whiting were susceptible to M. avidus infection under this thermal condition. At 24 °C, mortality remained low despite high growth potential observed in vitro, suggesting that Japanese whiting were less susceptible to disease at this temperature. These findings suggest that scuticociliatosis in Japanese whiting may be influenced by the balance between temperature-dependent growth of M. avidus and host physiological condition. Maintaining rearing water at approximately 24 °C may reduce the risk of scuticociliatosis under the experimental and rearing conditions examined in this study.